Medical catheters generally comprise elongated tube like members which may be inserted into the body either percutaneously, or via a body orifice, for any of a wide variety of diagnostic and therapeutic purposes. Such medical applications generally require the use of a catheter having the ability to turn corners, such as in ocular irrigation or aspiration applications, or to negotiate twists and turns, such as in certain cardiovascular applications.
Catheters are typically introduced to the patients body through an introducer sheath. The catheter must generally be straightened to fit through the introducer sheath. Therefore, the catheter must be constructed so that it is elastically resilient enough to go through the introducer sheath without plastic deformation, yet resilient enough to meet the performance needs of the particular medical procedure.
For some applications, an inner catheter having a preformed shape is straightened and placed in an outer guiding sheath. When the inner catheter is extended or the outer sheath withdrawn, the inner catheter assumes its original shape. Again, the inner catheter must be constructed so that it is elastically resilient enough to straighten without plastic deformation, yet resume its original configuration when the outer sheath is removed.
For example, percutaneous translumenal coronary angioplasty (PTCA) requires manipulation of a catheter from a proximal position outside the patient's body through branched or tortuous portions of the patients arterial system for purposes of alleviating an obstruction by inflating a balloon. This particular procedure has been performed with increasing frequency over the past years in preference to open heart bypass surgery.
FIG. 1 illustrates the typical configuration of a conventional left coronary guiding catheter 20 with a dilation balloon 24 in the aorta when engaged with a stenosis 24 in the left main coronary artery during the performance of left coronary artery PTCA. The application of force 22 to advance a dilation balloon across the region of stenosis 26 increases the bending stress 28 on the bend 30 of the guiding catheter 20. The pre-bent configuration of the guiding catheter 20, in this situation a left Judkin's configuration, is unable to overcome the resistance at the stenosis 24, causing distal end 32 to back away from the entrance of the left main coronary artery and the angioplasty balloon catheter 24 to prolapse in the accenting aorta, precluding further progress.
Inability to advance the angioplasty balloon across the coronary stenosis because of instability of the guiding catheter and subsequent prolapse of the angioplasty balloon catheter represents one of the most common reasons for failure during the performance of a coronary angioplasty procedure. The guiding catheter disengages in this circumstance because of its flexibility. The guiding catheter has intrinsic flexibility because it must conform to the configuration of the aorta and aortic arch, which contain both linear and curved segments, during introduction. Insertion of the guiding catheter requires that it be advanced over a guidewire up the aorta, which is relatively straight, and then over the aortic arch, which, as the name implies, is curvilinear.
The stability afforded by guiding catheters typically relates to the limited intrinsic stiffness of these catheters. The stiffness of these prior guiding catheters is subject to a "warm-up" phenomenon (becoming more flexible as they remain in the body and equilibrate to body temperature) and thus varies inversely with the temperature of the device. Hence, these catheters tend to be particularly stiff on introduction into the body, when flexibility is preferable, and yet relatively flexible and hence unstable following exposure to body temperatures during balloon catheter manipulation across a coronary stenosis when rigidity is preferable.
U.S. Pat. No. 4,909,787 (Danforth) discloses a catheter having a closed chamber eccentrically disposed along almost the entire length of the housing such that it virtually encompasses the housing. The catheter preferably contains a relatively elastic segment disposed preferentially along the outer circumference of the curvature of the catheter. The chamber may be filled with a fluid. The catheter is capable of asymmetric elongation when hydrostatic pressure is applied to the chamber, resulting in the development of bending stress and increased rigidity on the distal end as desired by the operator.
U.S. Pat. No. 5,456,674 (Bos et al.) discloses a catheter with variable longitudinal properties. The catheters are manufactured by simultaneously conveying a plurality of streams of different materials to a molding nozzle and merging the streams together to form a catheter. The catheter is manufactured with varying properties along its longitudinal axis corresponding to properties of the constituent streams of materials.